In today's wireless communication systems, a spatial multiplexing transmission technique is becoming widely used, in which a transmitting apparatus multiplexes and transmits different data streams in parallel through multiple transmitting antennas, and similarly, a receiving apparatus receives the data streams through multiple receiving antennas and demultiplexes the data streams, thus increasing a transmission capacity in proportion to the number of transmitting antennas. In addition, a standardization process of IEEE802.11n, which is a wireless LAN standard adopting the MIMO scheme, is in progress.
FIG. 7 shows a configuration of a conventional and typical MIMO communication system. A MIMO transmitting apparatus 401 is provided with a MIMO transmission data processing circuit 411 for converting an inputted data stream into a plurality m of data streams S1 to Sm; transmitter circuits 412-1 to 412-m for processes of the respective data streams S1 to Sm, such as data modulation, D/A conversion, quadrature modulation, upconversion, filtering, etc.; and antenna elements 413-1 to 413-m respectively connected to the transmitter circuits 412-1 to 412-m. Radio signals transmitted from the respective antenna elements 413-1 to 413-m are spatially multiplexed over a propagation channel represented by a matrix H=(hij), 1≦i≦n, 1≦j≦m, and the transmitted radio signals are received by a plurality n of antenna elements 421-1 to 421-n of a MIMO receiving apparatus 402. The MIMO receiving apparatus 402 is provided with the antenna elements 421-1 to 421-n; receiver circuits 422-1 to 422-n respectively connected to the antenna elements 421-1 to 421-n, for processes of the received radio signals, such as filtering, downconversion, quadrature detection, A/D conversion, demodulation, etc., to generate data streams X1 to Xn; and a MIMO receiving data processing circuit 423 for restoring one original data stream from the data streams X1 to Xn.
In the MIMO communication system, the MIMO transmitting apparatus 401 transmits known data (training information) to the MIMO receiving apparatus 402, and the MIMO receiving apparatus 402 estimates a propagation channel matrix H based on amplitude and phase information of the received data and the known data. As such, in the MIMO communication system, the communication quality is determined by not only electric field strengths of received signals, but also by phase differences between the signals, unlike a conventional SISO (Single-Input Single-Output) communication system. Hence, for example, even when moving one of the receiving antennas by one-half wavelength (about 3 cm at 5 GHz), its propagation channel changes, and accordingly, the communication quality also changes. Thus, it is important to appropriately design characteristics and arrangement of antennas.
Meanwhile, video streaming services are provided in transports with assigned seats, such as planes, long-distance trains, and buses, and in public facilities. It is advantageous to broadcast or multicast streaming data for such a service by using a MIMO communication system, since many users can enjoy a high quality video. Moreover, it is convenient for system installers to use a MIMO communication system, since the system installers do not have to experience difficulties in fixing and routing wires required for a wired communication system. FIG. 8 shows a schematic diagram of a wireless communication system for achieving data streaming using the MIMO communication scheme as described above. The wireless communication system of FIG. 8 includes one server apparatus 501 and multiple client apparatuses 502-1 to 502-5, and achieves its service by multicasting video/audio data by the server apparatus 501, and receiving and playing the video/audio data by each of the client apparatuses 502-1 to 502-5. By using the MIMO communication scheme in such service, it is possible to achieve video streaming with higher image quality. In this case, from the viewpoints of the manufacturing cost and the flexibility in installation, it is desirable that the multiple client apparatuses 502-1 to 502-5 have the same configuration, even their antennas are the same. However, there is a problem that the client apparatuses 502-1 to 502-5 have very different propagation channels depending on their installation locations, and the communication quality significantly degrades for some locations.
In order to solve such a problem, a selection diversity technique is commonly known in which each client apparatus is provided with multiple antennas, and receiving antennas are changed when the communication quality degrades. For example, a technique of Patent Literature 1 is known.
Citation List
Patent Literature
PATENT LITERATURE 1: PCT International Publication No. WO2005/004376.